Mail errors unknown address

I've not been able to receive any emails to the email i've setup using ispconfig. I can send no problem but there seems to be a problem receiving. When I email the account i get the following error back

5.10 - Unknown address error 550 - relay not permitted.

Seems i may have configured something incorrectly, can anyone help and i'm new to linux.

the server it resolved too is the internal IP address of the router here.

I dont really know what i should be setting my MX and @ records to with the registrar so that is my first problem! What should it be? At the moment the @ record is set to the external ip and the mx record is mx10.mydomain.co.uk